Since most plant leaves are green, it's apparent that they reflect green light; so, they absorbsunlight in the red and infrared. Phytochromes are used by plants in several ways. Angiosperms (flowering plants), detect light to regulate their flowering. They're used, also, to regulate the germination of seeds and the size and number of leaves.

A phytochrome molecule.

Phytochromes are proteins containing a bilinchromophore. A chromophore is the part of a molecule responsible for its color.

Phytochrome research is nearly a hundred years old. It began with studies in 1918 by botanist Harry A. Allard and physiologist Wightman W. Garner, both employees of the United States Department of Agriculture (USDA), who were working on a problem with soybeans. farmers, who tried to spread-out their soybean harvest by planting over a two week period, found that the plants would all flower at the same time.[1]

In a simple experiment, Allard and Garner grew some Biloxi soybeans in pots, leaving some outside all day long, but the others they placed in a dark shed every afternoon, bringing them back outdoors every morning. The soybeans, exposed to apparently shorter days, flowered five weeks earlier.

This research had an immediate benefit to florists, who were then able to get their flowers to bloom yearlong.[1] And there were still surprises. It was found that a single, thirty second burst of light in the extended period of darkness would prevent flowering.[1]

The plants were conditioned by first being grown at 16-hour days, followed by ten hour days to induce flowering. The plants were then briefly exposed to portions of the light spectrum in the middle of the dark period for six days. After a week of a regular long night cycle, they found that exposure to yellow and red light had the greatest affect on flowering.[1]

Land plant phytochromes detect shading by neighboring plants by the ratio of red to far-red light, and they change their development accordingly. About 20% of a plant's genes are regulated by phytochromes[2-3]. Says UC-Davis professor and senior author of the study, Clark Lagarias, "They control all aspects of a plant's life."[3] The phytochrome molecules are structurally related to chlorophyll.[2]

As shown in the figure, they found that these phytochromes detect light throughout the visible spectrum, sensing yellow, orange, green, and even blue light.[2] Since these different colors penetrate to different depths in water, such diversity offers an evolutionary advantage in that the algae can use whatever light is available. It appears that the ancestral form of phytochrome was sensitive to red light, and its structure evolved to accommodate the additional wavelengths of light.[3]